How to resize evenly?
The issue occurs when the object in scaled in object mode differently on several axis.

Here on left, an object that has (1, 1, 1) as scale (in object mode) and on right a scale of (1, 2, 1):



To avoid the non even scaling: go to object mode and apply the scale: CtrlA then choose 'scale'.

This kind of effects is true for several operations in Blender (scaling, beveling, extruding, etc.)

Why is it so: 

in edit mode the operation is effectively performed evenly, but after that the object scales are calculated, so the result is uneven.